1. Field of the Invention
The present invention concerns compressors for gas turbine engines.
2. Description of the Related Art
With reference to FIG. 1, a conventional ducted fan gas turbine engine generally indicated at 10 has a principal and rotational axis 11. The engine 10 comprises, in axial flow series, an air intake 12, a propulsive fan 13, an intermediate pressure compressor 14, a high-pressure compressor 15, combustion equipment 16, a high-pressure turbine 17, and intermediate pressure turbine 18, a low-pressure turbine 19 and a core engine exhaust nozzle 20. A nacelle 21 generally surrounds the engine 10 and defines the intake 12, a bypass duct 22 and a bypass exhaust nozzle 23.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 12 is accelerated by the fan 13 to produce two air flows: a first air flow into the intermediate pressure compressor 14 and a second air flow which passes through a bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 14 compresses the air flow directed into it before delivering that air to the high pressure compressor 15 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 15 is directed into the combustion equipment 16 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 17, 18, 19 before being exhausted through the nozzle 20 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines 17, 18, 19 respectively drive the high and intermediate pressure compressors 15, 14 and the fan 13 by suitable interconnecting shafts.
Alternative gas turbine engine arrangements may comprise a two, as opposed to three, shaft arrangement. In one known two-shaft configuration, the low-pressure turbine drives the fan only and the desired compression ratio into the combustor is achieved by a multi-stage high-pressure compressor. However it is generally desirable to increase the speed of the high-pressure compressor as far as possible in order to increase efficiency. The maximum speed that can be achieved by the high-pressure compressor is limited by the compressor blade tip diameter of the forward stages of the compressor, which have larger diameter than the smaller rearward stages. Furthermore the axial loads on the high-pressure spool for such a configuration are large.
Accordingly it is generally known that a so-called booster may provided in a two-shaft engine configuration in order to provide further compression of the core airflow in between the fan and the high-pressure compressor. The booster may driven by the low pressure turbine/shaft and thus rotates at the same speed as the fan. Such a booster has limited efficiency and offers only a limited compression ratio, requiring a large number of booster compressor stages, thereby carrying relatively large penalties in terms of cost, weight, engine length, and aerodynamic drag. The axial load relief for the high-pressure spool is also limited.
As a solution to the above deficiencies it is known to provide a booster driven directly by the low-pressure turbine but to provide a reduction gearbox between the booster and the fan. This allows the booster and fan to be driven at optimal respective speeds/efficiencies whilst also reducing the axial loading on the high-pressure spool.
However it will be appreciated by the skilled person that the aerodynamic efficiency of the compressors and turbines themselves is only one aspect of operational performance. There are a number of accessories that are typically required to be driven by the engine, comprising for example electrical generators, hydraulic pumps, fuel pumps and oil pumps. There is, in general, increasing demand for electrical power on airframes. However there exists a problem in that, unlike engine-dedicated accessories, the power demands of an airframe are independent of the engine operating conditions, such as the spool speeds. For example an aircraft may require a significant level of power even when the engine is idle or at low speeds.
Aircraft electrical power is conventionally generated by one or more accessory mounted generators such as an Integrated Drive Generator (IDG) and/or Variable Frequency Generator (VFG). The power to such generator(s) is extracted from the high-pressure shaft since the speed variation in that shaft is lower than that of the low-pressure shaft. IDG's feature an integral constant speed drive that ensures the generator operates at a fixed speed over the high-pressure range of operation, thereby ensuring a fixed electrical frequency output. For VFG's control electronics are used to correct frequency variations. The generators must be sized to ensure that the electrical supply meets the aircraft demands at the lowest engine speed settings.
This base level/requirement of power extraction requires that the high-pressure spool speed cannot drop below a lower cut-off value. That cut-off speed is typically higher than a desirable speed at engine idle, during taxi and/or descent during flight. Thus the high-pressure spool is operated at higher speeds to satisfy electrical demands, thereby reducing engine efficiency and increasing thrust when it is not needed (e.g. requiring the application of aircraft brakes on the ground and/or extending the descent phase during flight).